Simplify dynamic constant resolution to direct Shakapacker usage

Replace Object.const_get(ReactOnRails::PackerUtils.packer_type.capitalize)
with direct Shakapacker::Manifest::MissingEntryError since we only
support Shakapacker now. This removes unnecessary complexity and
improves code clarity.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/react_on_rails/utils.rb

@@ -94,9 +94,7 @@ def self.bundle_js_file_path(bundle_name)
       if ReactOnRails::PackerUtils.using_packer? && bundle_name != "manifest.json"
         begin
           ReactOnRails::PackerUtils.bundle_js_uri_from_packer(bundle_name)
-        rescue Object.const_get(
-          ReactOnRails::PackerUtils.packer_type.capitalize
-        )::Manifest::MissingEntryError
+        rescue Shakapacker::Manifest::MissingEntryError
           handle_missing_manifest_entry(bundle_name)
         end
       else